This invention relates generally to the rendering of latent electrostatic images visible on a charge retentive surface using multiple colors of dry toner or developer supplied by a plurality of developer housings, and more particularly, to controlling the charging level of a scorotron operably positioned between two developer housings.
The tri-level highlight color xerographic process is one method of making single pass, two color images. The basic concept of tri-level xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. In this process, the latent image is created by first charging the photoreceptor (p/r) negatively to some initial charge level (V.sub.o), and then exposing the p/r to three discrete voltage levels using a Raster Output Scanner (ROS). The two voltages that represent the document information (black and color) are commonly referred to as the Charged Area Development potential (V.sub.CAD) and the Discharged Area Development potential (V.sub.DAD). The third voltage represents the white or background potential (V.sub.WHITE), and corresponds to non-developed areas of a final print. V.sub.CAD is generated when the ROS output is minimum (off), and is roughly equal to V.sub.o. V.sub.DAD, on the other hand, is generated when the ROS output is maximum (on full), and is typically equal to the residual potential of the p/r (&lt;100v). V.sub.WHITE is generated when the ROS output is approximately at half power, and is typically equal to V.sub.0 /2.
Once the tri-level latent image is formed, it is then developed by passing it sequentially through or past two independent developer housings, each containing one of the two required developers. In theory, either of these housings can contain either a black or color developer, and either developer (specifically, the toner) can be either positive or negative in charge, as long as the two developers are opposite in polarity.
When the tri-level latent image is moved past the first developer housing, which by way of example is a DAD development system, the V.sub.DAD portion of the latent image is developed with one of the two toners. As development takes place, V.sub.DAD is changed in amplitude by a process called charge neutralization, which is the pairing of charges on the p/r with opposite polarity charges on the toner particles. In theory, total neutralization (100%) of V.sub.DAD is achieved when enough toner is deposited on the p/r to make the potential of the developed V.sub.DAD regions=V.sub.DAD bias. In practice, however, total neutralization is rarely achieved, and the Post Development (PD) V.sub.DAD voltage is typically 30 volts less negative then V.sub.DAD bias.
Development biases for the CAD and DAD development housings are typically set.about.100 volts above, and below V.sub.white respectively. Thus, the CAD positive toner at the second development station experiences a cleaning field of 100 volts to the background areas corresponding to V.sub.white, and because of the incomplete neutralization in the PD V.sub.dad, a larger cleaning field of 230 volts in those regions. These cleaning fields, coupled with the weakened magnetics (almost field free) employed in the second housing to minimize the disturbance of the developed DAD image, have the following undesired effects:
1. Because the CAD carrier beads are negative in charge, they are attracted to the more positive regions on the p/r surface. The most positive areas on the p/r prior to entering the second housing are the DAD (PD) areas, and as a result are the areas most likely to suffer deposition of CAD developer beads. The presence of beads in these regions results in large deletions in the DAD image when the image is transferred to paper. PA1 2. Because of the weak magnetics used in the second housing, the effective conductivity of the CAD developer is lower than it would be if it were in the first housing [which uses full strength (i.e. conventional magnetic brush development magnetics)]. As a result, the CAD developer is more likely to respond to fringe fields, such as the ones that exist between V.sub.DAD (PD) and V.sub.WHITE, causing black toner to be deposited around the outside of the DAD or color image areas. This type of deposition has been observed in the past on actual tri-level prints. PA1 3. Any wrong sign toner (negative) contained in the CAD developer will be attracted to, and possibly deposited in, the residual V.sub.DAD areas. While this may not be detrimental if the CAD developer is color toner because relatively small amounts of red toner in the black toner are not readily seen, it is very undesirable if the CAD developer is black.
Various techniques have been employed in the prior art to minimize the disturbance of the first developed image by developer materials in the another housing through which the developed image must pass. By in large such techniques have dealt with the modification of the development apparatus of the second developer system. For example:
There is disclosed in U.S. Pat. No. 4,308,821 granted on Jan. 5, 1982 to Matsumoto, et al, an electrophotographic development method and apparatus using two magnetic brushes for developing two-color images which do not disturb or destroy a first developed image during a second development process. This is because a second magnetic brush contacts the surface of a latent electrostatic image bearing member more lightly than a first magnetic brush and the toner scraping force of the second magnetic brush is reduced in comparison with that of the first magnetic brush by setting the magnetic flux density on a second non-magnetic sleeve with an internally disposed magnet smaller than the magnetic flux density on a first magnetic sleeve, or by adjusting the distance between the second non-magnetic sleeve and the surface of the latent electrostatic image bearing members. Further, by employing toners with different quantity of electric charge, high quality two-color images are obtained.
U.S. Pat. No. 3,457,900 discloses the use of a single magnetic brush for feeding developer into a cavity formed by the brush and an electrostatic image bearing surface faster than it is discharged thereby creating a roll-back of developer which is effective in toning an image. The magnetic brush is adapted to feed faster than it discharges by placement of strong magnets in a feed portion of the brush and weak magnets in a discharge portion of the brush.
U.S. Pat. No. 3,900,001 discloses an electrostatographic developing apparatus utilized in connection with the development of conventional xerographic images. It is utilized for applying developer material to a developer receiving surface in conformity with an electrostatic charge pattern wherein the developer is transported from the developer supply to a development zone while in a magnetic brush configuration and thereafter, transported through the development zone in magnetically unconstrained blanket contact with the developer receiving surface.
U.S. Pat. No. 4,833,504 granted to Parker et al on May 23, 1989 discloses a magnetic brush developer apparatus comprising a plurality of developer housings each including a plurality of magnetic rolls associated therewith. The magnetic rolls disposed in a second developer housing are constructed such that the radial component of the magnetic force field produces a magnetically free development zone intermediate a charge retentive surface and the magnetic rolls. The developer is moved through the zone magnetically unconstrained and, therefore, subjects the image developed by the first developer housing to minimal disturbance. Also, the developer is transported from one magnetic roll to the next. This apparatus provides an efficient means for developing the complimentary half of a tri-level latent image while at the same time allowing the already developed first half to pass through the second housing with minimum image disturbance.
It is known in the prior art to expose the charge retentive surface containing a developed image to corona discharge. As illustrated in U.S. Pat. No. 4,660,961 granted to Kuramoto et al on Apr. 28, 1987, a charging assembly is employed between two developer housings for providing additional uniform positive charge to the photosensitive surface used therein.
U.S. Pat. No. 4,562,130 granted to Tateki on Dec. 31, 1985 discloses the use of a scorotron device which is utilized for stabilizing an unstable intermediate potential on a charge retentive surface for the purpose of enabling the setting of developer bias voltages. The unstable potential area is raised to the grid voltage of the scorotron by exposure of the charge retentive surface to the scorotron discharges. The use of such a scorotron device is also disclosed in U.S. Pat. Nos. 4,525,447 granted to Tanaka on Jun. 25, 1985 and 4,539,2181 granted to Tanaka on Sep. 3, 1985.
U.S. Pat. No. 4,308,821 granted to Matsumoto et al on Jan. 5, 1982 disclose the differential charging of developer material in order to obviate materials interaction due to the stronger attractive forces of the one material and the charge retentive surface.
As disclosed in U.S. Pat. No. 4,486,089 granted on Dec. 4, 1984 to Itaya, et. al. a magnetic brush developing apparatus for a xerographic copying machine or electrostatic recording machine has a sleeve in which a plurality of magnetic pieces are arranged in alternating polarity. Each piece has a shape which produces two or more magnetic peaks. The sleeve and the magnets are rotated in opposite directions. As a result of the above, it is alleged that a soft developer body is obtained, and density unevenness or stripping of the image is avoided.
U.S. Pat. No. 4,868,611 granted to Richard P. Germain on Sep. 19, 1989 describes how a scorotron charging device, positioned between the first and second developer housings in a single pass, highlight color, tri-level xerographic system, can be used as a countermeasure against spurious induction development and carrier BCO occurring at the second development housing. The function of the scorotron is to raise the neutralization of a first developed image such that the potential difference between the neutralized image and the second developer housing bias voltage is below the threshold for the onset of BCO.